Control systems are widely used throughout the world. Such systems may be distinguished in their complexity from simple on/off temperature controllers (as would be found in a portable electric heater) to control systems of high complexity (as would be found controlling an entire petroleum refinery wherein many thousands of components are interconnected in order to manage the quality and quantity of product being produced).
Control systems may also be distinguished as centralized or distributed. These two distinguishable attributes of control systems—complexity and distributed nature—are themselves related because simple control systems are usually centralized, whereas complex systems often benefit from being distributed, if for no other reason because the systems are geographically large, as in the case of a petroleum refinery or electrical power grid. Control systems which are designed to operate in a distributed mode are called Distributed Control Systems (DCS). The above generalization notwithstanding, large geography is not the only driver for distributed control systems. For example, the substantial semiconductor fabrication tool market utilizes small machines whose high complexity requires large numbers of complex control components which can benefit from employing a distributed control architecture. The present invention is principally concerned with complex and distributed control systems and improvements thereto. However, the present invention applies equally well to the degenerate, non-distributed architecture.
While distributing a control system may simplify wiring and perhaps improve reliability, the act of distributing can cause difficulty in coordinating the actions of the various distributed pieces. Whereas there are many varieties of control systems, it is common to employ a Supervisory Control and Data Acquisition (SCADA) system to coordinate the various distributed control systems. As its name implies, the SCADA system does not usually perform the low-level control loop closure but rather the supervisory or coordinating role in keeping a large system running well. For example, in a petroleum refinery, a DCS can keep a given, local refining process stable whereas the SCADA system would adjust its rate and mixtures in order to keep this portion of the refining process synchronized with the rest of the many processes. The SCADA system performs this adjustment by modifying setpoints and other parameters in the DCS by communicating principally via a network connection, with the network connection generally inside of a Local Area Network (LAN) where communication timing, security and reliability is maintained.
Whereas SCADA systems rarely perform low-level loop control, the DCS itself may rely upon a lower-level control system such as Programmable Logic Controller (PLC). Indeed, in some more modern DCS systems, the PLC can play the role of a DCS. The PLC itself is made up of input/output modules (I/O modules) which connect to the sensors producing the information about the state of the process, and the actuators which provide the action of the control system. The prior art utilizes fixed-configuration I/O modules with multiple power supplies that necessitates custom wiring.
Referring to FIG. 1, we show three prior art fixed-configuration I/O modules, a first current input I/O module 21, capable of measuring industry standard 4-20 mA signals, a second frequency input I/O module 22, capable of counting pulses generated by a pulse device such as a water flow meter, and a third sourcing digital output I/O module 23, capable of driving contactors suitable for powering water pumps, for example. We note that these I/O modules are only three of many dozens of different I/O modules, such other variations being voltage output, current output, voltage input, digital PNP input, digital NPN input, and so forth. Each different variation of I/O module generally has unique wiring conventions, in general never directly from I/O module to sensor or actuator. Note the prior art connection means 20 for connecting an electrical conductor carrying an electrical signal or electrical power to a device, for example a pressure transducer 24. The device may be a sensor, actuator, power supply or I/O module. As is customary in the art, we employ any of a number of connection means 20 such as but not limited to terminal blocks, pluggable terminal blocks, crimped wire/connector devices with housings, circuit board mounted connectors and connectors utilizing soldered or welded joints. Connection means 20 is required at many places in any control system, and occurs one or more times with each device to effect the connection of signals and power to the sensors and actuators, for example when there is a distance of many meters employing multiple wire or cable systems. Also in FIG. 1 we show five devices, three sensors and two actuators as would be common in a water pumping application. Sensors for well depth 24 and pump electrical current 25 are presented to the I/O module as industry standard 4-20 mA signals which notably do not connect directly to the appropriate current input I/O module 21 but rather through some custom wiring and components 30 including terminal blocks 31 and device power supply 32, which is in addition to the module power supply 33. Connected to the frequency input I/O module 22 is a pulse generating sensor 26 such as is common in measuring water flow. The pulse generating sensor 26 requires one more wire than the two previous sensors 24 and 25. Note that only one of the three wires from the pulse generating sensor 26 is connected to the corresponding frequency input module 22. The other two wires connect utilizing custom wiring and components 30. Finally, two actuators, identical motor contactors 27, are wired to a sourcing digital output I/O module 23. As was the case with the sensors, the connection of the two actuators 27 to the I/O module 23 is not direct but also requires custom wiring and components 30.
FIG. 2 depicts the prior art component elements required to implement one node of a DCS control system 41. The DCS node 41 is made up of a PLC 40, three I/O modules 21, 22 and 23 and custom wiring and components 30 which include terminal blocks in addition to device power supply (shown as 31 and 32 in FIG. 1). Sensors and actuators 24, 25, 26 and 27 connect via custom wiring and components 30.
FIG. 3 depicts the prior art SCADA system 43 made up of three distributed nodes 41 connected inside of the LAN. The distributed nodes 41 are connected—employing either a physical connection or a software connection—via peer-to-peer connections 28 one to another. In addition, the distributed nodes 41 are connected in a polling arrangement to a SCADA computer 42, normally inside of the LAN in order to provide good coordinating control among the distributed nodes 41.
Thus a completed SCADA system 43 utilizing one or more DCS components 41 must provide for wiring of the sensors such that the control system may measure with the sensors, compute based upon what it is instructed and what it measures, and then act to open valves, turn on pumps and heaters, utilizing actuators connected to fixed configuration output modules by way of custom wiring and additional power supplies.
The principal method of extracting information from the lowest level sensors via their associated I/O modules involves polling. Polling is accomplished by the higher-level control component as it sends a message requesting a specific value, sometimes called a tag. Polling is an active process that occurs from the upper level of the control system toward the lower level of the control system. Thus the PLC will poll the I/O module for the state of a sensor. The SCADA system will then poll the DCS or PLC system in order to retrieve that same sensor state or a combination of sensor states so that this information is available at the top level, the SCADA computer 42. The same process is employed to change the state of an output. The upper level control component sends a similar message to the lower level device instructing it to effect some kind of change to an output, either setting a level or turning it off or on.
An important operating mode of modern prior art SCADA, DCS or PLC systems is for the higher level control elements to initiate the communication with the lower levels, the levels being numerous, involving the I/O modules, the PLC, the DCS and the SCADA system.
There are exceptions to this operating mode, for example but not limited to, the use of alarms where an asynchronous event can cause a message to be sent from the DCS to the SCADA system. However, this exception does not change the fundamental and predominant mode of operation being initiated at the upper level and directed to the lower level.
Prior art SCADA, DCS or PLC systems are complex collections of many parts. At the lowest level, the sensor and actuator interface, the prior art systems employ largely fixed configuration I/O modules, thus separate products or separate product permutations are required in order to deliver many electrical interfaces such as 4-20 mA, +/−10V, Frequency, Level, NPN, PNP, sensor power and so forth. Multiple I/O modules are therefore commonly required. The multiple I/O modules typically plug into some bus interface. Sensor and actuator wiring require power supplies, terminal blocks and many wires.
Prior art SCADA 43 or DCS 41 systems are themselves layered by levels such that the SCADA computer 42 is connected via a network to one or more DCS systems 41 which may either be a PLC 40 or the DCS 41 connected via another layer to a PLC 40. Many hardware components are therefore required to implement a SCADA 43 or DCS 41 system.
On the software side, prior art I/O modules, e.g. 21, 22 and 23, in general, perform the electrical interface function, whereas the device-level software is handled in the PLC 40 or DCS 41. Device-level software includes software for linearizing, filtering, counting, differentiating, calibrating, enabling, sequencing and scaling the electrical value to engineering units. In order for a PLC 40 or DCS 41 to compute a pulse rate, for example, it is required to poll and input I/O module for the current value of the pulse input and then precisely time its period and track its frequency. Errors in computing a precise and accurate rate are very sensitive to measurement timing, thus delays in the polling by the PLC 40 or DCS 41 can negatively affect sensor measurement quality.
In summary, the prior art SCADA 43 and DCS 41 systems are highly complex, being made up of many different components with layers conducting top-to-bottom polling to bring information and control up to the upper layers of the control system.
SCADA systems 43 traditionally run on a standard computer platform such as a PC running a commercial operating system such as Microsoft Windows-XP or Microsoft Windows-7 or Microsoft Windows-8. DCS systems 41 also make extensive use of this PC architecture. Because the DCS 41 and often SCADA system 43 require tight and reliable connection to the process, these PC computers must be localized on-site, sometimes removed from offices where the PC can be more easily supported. Such localization removed from office networks, is often required to improve network determinism or predictability of response given the aforementioned effect that timing has on measurement accuracy.